A 750bp EcoRI fragment defining the minimal vg boundary enhancer (BE) is inserted in -750 to -1 orientation upstream of a minimal Hsp70 promoter driving expression of a chimeric protein which consists of the first 20 amino acids of vg, joined via one copy of the Tag:HA tag to the GFP coding region, followed by two Tag:HA tags, joined to the rest of the vg coding region at amino acid 25.